The present invention relates to constant temperature anemometers, and more particularly to the control circuitry for adjusting the electrical response in such anemometers upon sensing departures from a predetermined sensor operating temperature.
Thermal anemometers are used to sense highly localized velocities in fluid flows. A minute sensing element is placed in the fluid flow, and heated to a temperature above that of the fluid. The element loses heat to the fluid at a rate that increases with increases in the fluid velocity, assuming of course that other characteristics such as fluid temperature remain constant. Thermal anemometry frequently is called "hot wire anemometry" or "hot film anemometry", reflecting the types of sensing elements most frequently employed. Wire sensing elements have diameters on the order of 5-15 microns, typically formed of tungsten coated with platinum; platinum; and platinum/iridium alloys. An example of a film sensor is a platinum thin film applied to a fused quartz substrate.
The sensing element material must have a resistance that varies with temperature. For most such materials, resistance increases linearly with temperature according to the equation: EQU R.sub.s /R.sub.o =1+.alpha.(T.sub.s -T.sub.0) (1)
where R.sub.0 is the sensing element resistance at a reference temperature, e.g. either zero degrees C. or room temperature; T.sub.0 is the reference temperature; T.sub.s is the sensor operating temperature; and .alpha. (alpha) is the temperature coefficient of resistance. R.sub.s /R.sub.o is known as the "overheat ratio". As one example, tungsten has coefficient .alpha. of 0.0042 ohms/.degree.C.
In a conventional constant temperature anemometer (FIG. 1), a sensing element 16 is incorporated into an arm 18 of a Wheatstone bridge 20 having three other arms indicated at 22, 24, and 26. Between the arms are several junctions or terminals, including a drive terminal 28, a base terminal 30, and two intermediate terminals 32 and 34. Base terminal 30 is maintained at ground. Drive terminal 28 is biased to a drive voltage that is variable to control the current through the bridge, and more specifically current through sensing element 16 to control resistance R.sub.s. Fixed resistors 36 and 38 are provided along arms 22 and 24, respectively. Along arm 18 is a "resistor" 40 that does not vary with temperature. However, because it represents a resistance R.sub.c of components that accompany the sensing element (such as probe, support and cable), it is subject to change whenever one sensing element is substituted for another. Likewise, an inductance 42 varies with different probes, etc. A capacitor 43 represents the capacitance of these same components. Accordingly, an adjustable resistor 44 and an adjustable inductor 46 are provided along arm 26.
Fixed resistors 36 and 38 determine the bridge ratio. The bridge ratio (resistance of 38 divided by resistance of 36) can be 1:1, but frequently is 5:1 or 10:1 to channel the greater share of the current through the "active" side of the bridge, i.e. arms 18 and 22. The adjustable resistor 44 and inductor 46 for tuning the bridge are preferably on the passive side of the bridge (arms 24 and 26).
The components of the bridge are selected and adjusted so that when sensing element 16 is at the selected operating temperature, resistance R.sub.3 of resistor 44 as compared to the combined resistance R.sub.c and R.sub.s replicates the bridge ratio, with the result that the voltages at intermediate terminals 32 and 34 are the same. A fluid velocity decrease causes an increase in sensor temperature and increases R.sub.s, thus to increase the voltage at terminal 32. An operational amplifier 48, responsive to sensing the bridge imbalance, generates a reduced voltage as its output, i.e. the drive voltage is reduced. This reduces current along both the active and passive sides of the bridge. The reduction in current through sensing element 16 decreases its temperature, and thus decreases its resistance.
Conversely, an increase in fluid velocity removes heat from the sensing element more rapidly, decreasing its temperature, whereupon the reverse sequence occurs. In either event, amplifier 48 responds to a sensed imbalance in the bridge, to drive sensing element 16 back toward the selected operating temperature and resistance. The drive voltage thus is a measure of the rate at which the fluid removes heat from the sensing element. Assuming constant temperature (or a means to compensate for fluid temperature changes), the heat removal rate is a direct function of fluid velocity.
Constant temperature (or constant resistance) anemometers are considered superior to constant current systems in measuring turbulent or unsteady flows, due to substantially reduced sensor temperature variation in response to a given change in fluid velocity. The small size, high frequency response and low noise characteristics of the wire and film sensors facilitates their use in measuring turbulent flows. The constant temperature approach also is considered advantageous because of the substantially flat frequency response over a wider range of fluid velocities without the need for adjustment, and the relative ease in compensating for changes in fluid temperature, either by incorporating a temperature sensitive element in the bridge or by measuring and compensating for temperature.
A disadvantage of the constant temperature anemometer is the need to optimize frequency response of the system, primarily by adjusting the operational amplifier gain and the bridge reactance. The amplifier gain in the constant temperature anemometer is valid only for a particular flow velocity (i.e., a particular sensor heat loss rate) and a limited range about that velocity. Deviations from that velocity can lead to significant error and cause self-excitation in the system. Accordingly, N. B. Wood (A Method for Determination and Control of the Frequency Response of the Constant-Temperature Hot-Wire Anemometer, Journal of Fluid Mechanics, 1975, Volume 67, Part 4, pp. 769-786) observes that frequency response is affected by changes in flow conditions. Wood found the frequency response to vary with the mean bridge voltage, charting an increase in preferred amplifier gain setting from 145 to 760 as mean bridge voltage ranged from about 4.2 to about 1.5 volts. Based on this relationship, Wood advised operators to observe the mean bridge voltage under operating conditions and set the amplifier gain based on the measured value of mean bridge voltage. Besides adjusting gain, it is known to reduce the bandwidth of the operational amplifier and reduce feedback effects.
Wood also reported that a change in the overheat ratio affects the appropriate gain setting of the operational amplifier. The overheat ratio can change due to a difference in fluid temperature or substitution of another sensing element.
Sensing element substitution gives rise to the need to adjust bridge reactance to match the probe cable and associated wiring. Resistance R.sub.3 and inductance L.sub.2 of inductor 46 are adjusted to tune the bridge to the particular sensor, probe, cable and flow conditions.
The need for these adjustments adds to the difficulty of using the constant temperature anemometer and severely limits its utility in monitoring turbulent, unsteady flows.
Therefore, it is an object of the present invention to provide a constant temperature anemometer with a broader operating frequency bandwidth and a more consistent frequency response over widely varying fluid velocities.
Another object is to provide, in a constant temperature anemometer, a means for automatically adjusting operational amplifier offset based on a detected imbalance in the bridge circuit.
Yet another object is to provide a bridge circuit incorporating a resistance that varies with a measured parameter, in combination with a means for continually optimizing the gain of an amplifier that controls the current through the resistance.